Electronic up-conversion: a combination of the advantages of impurity photovoltaics and (optical) up-conversion

In this paper we discuss ´electronically coupled up-conversion´ (ECUC) a new concept for improving solar cell efficiency, which is closely related to impurity photovoltaics (IPV) and (optically coupled) up-conversion (OCUC). The main advantages of ECUC over IPV are the same as those of OCUC over IPV: The solar cell is separated from the up-conversion process and is therefore only minimally influenced by the up-conversion. Furthermore, ECUC allows in the same way as OCUC a simple realization of natural photon energy selectivity. On the other hand, the main difference between ECUC and OCUC is that in ECUC the stepwise-excited electron-hole (e-h) pairs are injected directly into the solar cell without a ´detour´ via radiative recombination. Due to this difference the demanding requirements regarding the radiative efficiency of the up-converter material do not apply. We develop an idealized detailed balance description of the ECUC concept and calculate the corresponding optimum energy thresholds and efficiency limits. In particular we calculate the idealized ECUC efficiency for a crystalline silicon solar cell and an electronic up-converter made of amorphous silicon. Furthermore we discuss the impact of non-radiative recombination.